Y. He et al. /Ceramics Intemational 34(2008)1399-1403 Fig. 4. SEM image of Mgo whiskers prepared by conversion of Mg(OH)2 whiskers mechanism in hydrolysis of Mg and by a Vs mechanism in hydrolysis of MgCl2[8]. Since there are no globules found at na hanism taking place in our experiments. shows the SEM images of the product prepared by conversion of magnesium hydroxide whiskers. It has been found that they are completely composed of magnesia whiskers with diameter of 0. 1-0.4 um and length of 3-10 um. Whiskers eem uniform and straight. Taking a close look it could be found 20 W SO Mooo SEt WI that thick whiskers are consisted of several thin whiskers Fig 5 shows the HRTEM images of the product prepared by conversion. Fig. 5(a)and(b)show that whiskers obtained by Fig 3. SEM images of product prepared by hydrolysis of melt. conversion often have a length-to-diameter ratio of 30 or more The diameter of thin whiskers is not uniform and ranges from greatly. Some platelets and whiskers with branches are found 10 to 200 nm. The surface of whiskers is rough. Short crystals and most whiskers are found growing in bundles. A large part of may be broken whiskers during ultrasonic dispersion before whiskers have kinks and uniform diameter. Dispersed short HRTEM study. Fig. 5(c) shows that the interlayer spacil whiskers may be due to mechanical damage. There are two 0.24 nm and the growth direction is about 54 degrees with the ways to verify whether products are single crystal or not, X-ray interlayer. It was deduced that the growth direction is (11 0) diffraction and electron diffraction, which demands the size of Electron diffraction pattern in Fig. 6 reveals that the single crystals to be larger than 10 um and less than 1 um, crystal structure can be retained after conversion. respectively. Unfortunately, the size of the whiskers fall beyond these ranges. It is difficult to verify whether they are 3. 2. Comparison of whiskers made by different methods single crystals or not in our experiments. Nevertheless, it ha been reported that small magnesia whiskers prepared in this The Xrd patterns show that the purity of the product way are single crystal with a growth direction of [100][6] repared by hydrolysis is better than that prepared by The yield of hydrolysis of melt is often less than 40%. conversion of magnesium hydroxide whiskers. From the Considering that MgCl2-KCI has a marked vapor pressure of images it can be found that magnes species containing MgCl2 at 900C[7] and the water vapor hydrolysis are thicker and longer. The surface is smoother and pressure is low, the reaction probably took place in the vapor the size is more uniform. But except the whiskers, some phase and some products were carried away by the gas flow. platelets and grains are also found in the hydrolysis product. It When a crucible was put near the alumina boat in the down- is difficult to separate them from whiskers. The morphology of stream direction, white deposit was found on the crucible. So the product by conversion is more uniform but irregular the fact that the reaction took place in the vapor phase was diameters are often found verified. The Vapor-Liquid-Solid (VLS)and Vapor-Solid (VS) The hydrolysis method is simple, efficient and low-cost, but mechanisms are usually used to explain the growth of whiskers the quality of product is difficult to control. Though it has been from vapor. The growth mechanism of magnesia whiskers by studied for many years, industrial production of magnesia hydrolysis is only reported in the cases of hydrolysis of whiskers by this method has never been reported Large part of magnesium chloride with no additives and hydrolysis of Mg magnesia whiskers is formed in bundles and it is difficult to 8]. It is believed that magnesia whiskers grow by a Vls disperse them. The yield was often less than 40% in ourgreatly. Some platelets and whiskers with branches are found and most whiskers are found growing in bundles. A large part of whiskers have kinks and uniform diameter. Dispersed short whiskers may be due to mechanical damage. There are two ways to verify whether products are single crystal or not, X-ray diffraction and electron diffraction, which demands the size of crystals to be larger than 10 mm and less than 1 mm, respectively. Unfortunately, the size of the whiskers falls beyond these ranges. It is difficult to verify whether they are single crystals or not in our experiments. Nevertheless, it has been reported that small magnesia whiskers prepared in this way are single crystal with a growth direction of [1 0 0] [6]. The yield of hydrolysis of melt is often less than 40%. Considering that MgCl2–KCl has a marked vapor pressure of species containing MgCl2 at 900 8C [7] and the water vapor pressure is low, the reaction probably took place in the vapor phase and some products were carried away by the gas flow. When a crucible was put near the alumina boat in the down￾stream direction, white deposit was found on the crucible. So the fact that the reaction took place in the vapor phase was verified. The Vapor–Liquid–Solid (VLS) and Vapor–Solid (VS) mechanisms are usually used to explain the growth of whiskers from vapor. The growth mechanism of magnesia whiskers by hydrolysis is only reported in the cases of hydrolysis of magnesium chloride with no additives and hydrolysis of Mg [8]. It is believed that magnesia whiskers grow by a VLS mechanism in hydrolysis of Mg and by a VS mechanism in hydrolysis of MgCl2 [8]. Since there are no globules found at the end of whiskers, VS may be the mechanism taking place in our experiments. Fig. 4 shows the SEM images of the product prepared by conversion of magnesium hydroxide whiskers. It has been found that they are completely composed of magnesia whiskers with diameter of 0.1–0.4 mm and length of 3–10 mm. Whiskers seem uniform and straight. Taking a close look it could be found that thick whiskers are consisted of several thin whiskers. Fig. 5 shows the HRTEM images of the product prepared by conversion. Fig. 5(a) and (b) show that whiskers obtained by conversion often have a length-to-diameter ratio of 30 or more. The diameter of thin whiskers is not uniform and ranges from 10 to 200 nm. The surface of whiskers is rough. Short crystals may be broken whiskers during ultrasonic dispersion before HRTEM study. Fig. 5(c) shows that the interlayer spacing is 0.24 nm and the growth direction is about 54 degrees with the interlayer. It was deduced that the growth direction is h110i. Electron diffraction pattern in Fig. 6 reveals that the single crystal structure can be retained after conversion. 3.2. Comparison of whiskers made by different methods The XRD patterns show that the purity of the product prepared by hydrolysis is better than that prepared by conversion of magnesium hydroxide whiskers. From the images it can be found that magnesia whiskers made by hydrolysis are thicker and longer. The surface is smoother and the size is more uniform. But except the whiskers, some platelets and grains are also found in the hydrolysis product. It is difficult to separate them from whiskers. The morphology of the product by conversion is more uniform but irregular diameters are often found. The hydrolysis method is simple, efficient and low-cost, but the quality of product is difficult to control. Though it has been studied for many years, industrial production of magnesia whiskers by this method has never been reported. Large part of magnesia whiskers is formed in bundles and it is difficult to disperse them. The yield was often less than 40% in our Fig. 3. SEM images of product prepared by hydrolysis of melt. Fig. 4. SEM image of MgO whiskers prepared by conversion of Mg(OH)2 whiskers. Y. He et al. / Ceramics International 34 (2008) 1399–1403 1401